Since the announcement of the first strong GaN blue-color, light-emitting diodes (LEDs), the interest in GaN photonics and electronics has grown steadily, and the commercial applications have expanded to the extent that GaN devices now comprise a viable industry. The key step forward was the development of a high-quality p-type GaN epitaxial layer using Mg as a dopant and an AlN buffer layer on a sapphire substrate. While the n-type dopant Si in GaN manifests as a shallow donor (˜15 meV), p-type dopants, such as Mg in GaN, manifest as much deeper acceptors (˜160 meV). The sapphire substrate was used instead of a GaN substrate because high-quality GaN substrates were not available at that time. Nevertheless, it allowed the growth of a traditional p-n (homo) junction LED having qualities similar to those demonstrated in GaAs since the 1960s. An exemplary conduction-valence band-bending plot is shown in FIG. 1(a). The LED emission wavelength and external quantum efficiency (QE) were around 430 nm (violet: 380-450 nm) and 0.2%, respectively, the latter being much higher than the QEs reported for previous blue-emitting LEDs based on SiC and ZnSe LEDs. However, the peak wavelength was considerably longer than the expected cross bandgap (UG=3.4 eV) wavelength of 360 nm. This indicated that the recombination was via impurity states, likely associated with the p-type Mg dopant.
Soon thereafter GaN-based quantum-well LEDs were demonstrated using InGaN for the quantum well. Although the external QE was only 0.15% and the emission wavelength was still in the violet at 415 nm, this was considered a major step forward since the use of a quantum well allows for tuning of the emission wavelength through control of the In fraction and the well width. GaN/InGaN quantum well LEDs were demonstrated ranging in peak emission wavelength from blue around 450 nm (range=450-495 nm) to red at 675 nm (range=620-740 nm). In addition, for the blue emitters an external QE of 20% was achieved.
The GaN/InGaN LED development segued quickly into the GaN laser diode (LD), demonstrated first in 1996. The gain medium consisted of multiple InGaN quantum wells and the lasing wavelength was near 404 nm (violet). However, the laser cavity consisted of the traditional in-plane double-cleaved-facet structure, so the emission occurred in the same plane as the quantum wells, not the more desirable vertical direction. So researchers pursued the vertical cavity, surface emitting laser (VCSEL) diode. However, the same issue that plagued GaN LEDs from the beginning—the high resistivity of the p-doped GaN (e.g., Mg dopants)—again became a problem. This is because LDs of all types generally run at higher electrical current levels than LEDs since more current is required across the p-n junction to generate the electron-hole population inversion necessary for lasing action. The resistive p-doped layer not only causes a significant voltage drop and Joule heating, but it also creates a non-uniformity in the electric potential which is deleterious to the laser efficiency. This is in addition to the fact that the p-type GaN is generally difficult to grow, requiring extra materials (e.g., magnesium dopant) and processing (e.g., high-temperature rapid thermal annealing to activate), which also adds cost and reduces the yield in fabricating both LEDs and LDs alike.
The p-doping challenge has led researchers to unusual methods to mitigate the p-doping issues. To achieve population inversion and uniform light output across a LED to mitigate current crowding, the electrical pump current needs to be uniformly spread over the p-GaN contact area, starting from ohmic contacts located outside the optical cavity defined by the DBR mirrors. The high resistivity of the p-GaN region becomes a bottleneck for uniform carrier spreading. To overcome this problem, a thin highly conductive ITO layer was introduced to reduce the resistance. However, the ITO layer adds additional difficulties in deposition and fabrication, and can contribute non-negligible loss to the optical cavity, which leads to higher threshold current.
Independent of the detail design, all conventional p-n-junction GaN-based light emitters suffer from a phenomenon called current “droop”. This occurs in devices designed for intense light emission, whereby the emission strength and internal quantum efficiency fall with increasing current density above a certain level. The physical reason is the poor mobility and high resistivity of the holes in p-doped GaN. This causes the p-doped regions to heat up with increasing current density, which in turn increases the resistivity further and causes a significant fraction of the bias voltage to drop across the p-type region.